The present invention generally relates to interfacing between devices having different operating voltages, in particular, signal level interfacing between mobile phones and their associated accessories.
A continuing trend in portable devices is to provide devices with lower and lower operating voltages. Newer equipment and ASICs have been designed so that they may now run at much lower operating voltages. Lower operating voltages have many desirable benefits such as the need for fewer cells in a battery pack for portable devices. Operation at lower voltages can also improve speed, packaging density, and efficiency while reducing heat generation. Traditionally, most portable devices, such as mobile phones, have operating voltages of .about.5 volts and digitally communicate at a 5 volt logic-level. However, it appears that the traditional 5 volt-logic technology is becoming increasingly displaced by newer circuitry that is designed to operate at .about.3 volts supply voltage. For example, many existing mobile phones utilize 5 NiMH (Nickel Metal Hydride) cells, that correspond to a phone operating voltage of up to .about.5 volts. In contrast, newer phones and their accessories are now being designed to utilize 3 NiMH cells or one Li-ion cell, that correspond down to an operating voltage of .about.3 volts. As more systems adopt the three volt logic, a resulting problem presented to the designer is how to convert between three volt logic of newer mobile phones and the five volt logic of the previously existing accessories. Of course, one skilled in the art will recognize that this problem is not restricted to 3 and 5 volt logic level devices but can exist between any devices that communicate digitally with each other and that operate at different voltage levels.
One possible solution would be to provide a switched-mode voltage supply in the accessory or mobile phone. When the accessory (or mobile phone) was connected to a mobile phone (or accessory), the switched-mode voltage supply could be set to an appropriate voltage supply mode based upon the sensed voltage levels of the connected device. However, observation of available switches indicates that an operating efficiency of not much over 70% can be expected when working from, for example, a 5 volt input, delivering about 5 amps. Thus, a problem with this solution is that the voltage loses associated with the power switch, for example those across a free wheeling diode and associated the rectifying diodes, are too large a fraction of the 5 volts. This, in turn, may cause the resulting voltage to be at least a logically indetermination level. This problem is further aggravated by the relatively high current load involved.
Another possible solution would be to use a linear regulator to adjust the voltage of a fixed 5 volt supply to three volts when a connected device operated at the lower logic level. However, considering additional factors, such as electrical noise and circuit complexity, using a linear regulator becomes considerably less attractive. For example, the operating efficiency of a linear regulator used to convert 5 volt to 3 volt is only about 60%. It is therefore apparent that using a linear regulation in combination with a fixed voltage supply is even less desirable than using a switched mode voltage supply to solve this problem.
As portable devices, such as mobile phones or lap tops, switch to different operating voltages, more and more existing devices will be incompatible with newer devices and their accessories. In the case of mobile phones, newer mobile phones designed to operate at the 3 volt operating level will be incompatible with existing 5 volt operating level accessories and vice versa. As a result, there will be an increased need for signal level interfacing of the digital information that is to be communicated between devices in order for the newer devices and their accessories having different operating voltages and logic to be compatible.